Modem optical communication systems utilize optical amplifiers to amplify wavelength division multiplexed (WDM) signal channels as they are transmitted through the system. One can differentiate between two types of amplifiers commonly used in such systems:
1. Lumped amplifiers, which are self-contained units placed at certain points along the transmission link, with the signal amplification occurring wholly within the amplifier itself. The most common example of such a lumped amplifier is the Erbium doped fiber amplifier (EDFA), which contains a special Erbium doped fiber (EDF). The EDF serves as a gain medium used to transfer energy from laser diode pumps within the amplifier to the optical signal channels as they pass through the amplifier.
2. Distributed Raman amplifiers (DRAs), in which (in contrast with EDFAs) the transmission fiber itself serves as the gain medium, meaning that the signal channels are amplified as they travel through the transmission fiber. In DRAs, the amplification process is distributed along the transmission fiber, as opposed to being lumped in a self-contained unit as in the case of the EDFA. This allows the distance between EDFAs to be increased beyond 120 km, and/or the optical signal to noise ratio (OSNR) of the system to be improved, thus allowing higher bandwidth communication.
FIG. 1 shows an optical communication system employing a lumped optical amplifier as known in the art. The communication system comprises a WDM transmitter 102 which can transmit a WDM signal comprising one or more WDM channels within a specified transmission band (for example, the C-Band, 1525-1565 nm). The transmitted W)M signal propagates along a transmission fiber span 104, being attenuated as it propagates. Lumped optical amplifier 106 re-amplifies the signal, which then continues to propagate along a transmission fiber span 104′ until it is received at a WDM receiver 108. The communication system may contain additional transmission fiber spans, as well as additional lumped optical amplifiers placed before or after the spans. In addition the communication system may also employ DRAS.
One major requirement of all optical amplifiers is that they comply with various laser safety standards (such as International Standard, “Safety of Laser Products—Part 1: Equipment Classification, Requirements and User's Guide”, IEC 60825-1 and International Standard, “Safety of Laser Products—Part 2: Safety of Optical Fiber Communication Systems”, IEC 60825-2). These standards specify a certain limit for safe (so called “class 1M”) radiation, such that exposure to such radiation under normal circumstances will not cause eye or skin damage. For example, for radiation in the wavelength region >1400 nm, this limit is about 120 mW (21 dBm). If an amplifier has higher output power, for example in the case of a DRA or a lumped amplifier with high output power, then an automatic shut-down procedure needs to be provided in order to retain a class 1M safety classification. This means that if a disruption occurs in the fiber link (e.g. due to open span) the amplifier will shut-down or reduce output power to a safe level, thus avoiding potential hazard to technicians and equipment. As used herein, the term “open span” refers to the state where there is an open connector, fiber break or cut within the span connected to the output port of an optical amplifier, or any other situation that may cause significant leakage of optical power from the span, thereby posing danger to human eyes coming in contact with the leaked power. The term “opening” is used to refer to the point along the span where the leakage of power occurs. There is clearly a need to immediately and automatically detect any such open span, and shut down the optical amplifier a time span short enough to avoid harm to human eyes (henceforth referred to as “eye-safe time”).
The automatic shutdown mechanism should on the one hand be as fail safe as possible, and on the other hand not be activated mistakenly by events that do not pose potential safety hazards. Another desired feature is that the shutdown mechanism be a local and autonomous integrated feature of the amplifier, to further enhance safety and to avoid dependence on other parts of the communication system. Finally, the detection system should ideally provide as much information as possible to the system management with regard to the type of failure (e.g. fiber break or open connector), and its position along the span. This facilitates rapid correction of the failure, and minimization of system downtime.
One method known to the art for providing autonomous and local automatic shut-down of an amplifier is based on monitoring the signal back-reflection that enters the output port of the amplifier. If a connector of type PC (i.e. Polished Connector without an angle) is open down-stream of the output port, then the signal may be strongly back-reflected from this open connector and the strong back-reflection detected upon entering the output port of the amplifier, thus leading to the detection of the open connector. The main problem with this method is that it works only when the open span is due to an open connector of type PC, which causes strong back-reflection. When the open span is due to a fiber break or due to an open connector of type APC (Angle Polished Connector) the back-reflection is small, and will typically be masked by other back-reflections (e.g. Rayleigh scattering, or scattering from splices and closed connectors) normally occurring in the transmission link, thus preventing detection of the open span.
U.S. patent application Ser. No. 11/464,198 discloses a system and method for providing eye-safety protection during operation of distributed Raman amplifiers based on continuous monitoring of out-of-band amplified spontaneous scattering (ASS) created in the transmission span coupled to the Raman amplifier, and real-time detection and analysis of changes in the monitored ASS power level. This method has the advantage that it can detect any type of open span, not just an open span due to a PC connector. The system includes at least one Raman pump for introducing Raman energy into the span, a monitoring unit for performing the continuous ASS monitoring, and a control unit operative to detect and analyze in real-time changes in the ASS power, and upon determination that such changes indicate an open span, to reduce the level of the Raman pump energy entering the span to a safe level.
The method described in U.S. patent application Ser. No. 11/464,198 is naturally suited to distributed Raman amplifiers, since a DRA, by design, outputs high pump power which purpose is to cause Raman amplification in the transmission fiber. The wavelength of each pump and the pump power are controlled by the amplifier itself and typically remains constant. Thus, the level of ASS directed back to the DRA and its decrease due to an open span can be relatively well predicted. In contrast, lumped amplifiers such as EDFAs do not output pump power to the transmission line, but instead output an amplified signal. The spectral composition of the amplified signal is typically not known by the amplifier unit, and more importantly, can change suddenly due to signal Add/Drop events occurring in the optical link. Thus, while the high power amplified signal also generates Raman ASS in the transmission fiber (ASS which is directed back to the amplifier and can be detected), the magnitude and characteristics of this ASS are highly unpredictable and can change suddenly with time unrelated to an open span event.
Accordingly, there is a need for, and it would be advantageous to have a system and method for improved eye protection safety of high power lumped optical amplifiers deployed in networks where dynamic changes such as Add/Drop can occur.